* Move as much as possible to the CSS in the template.
* Ensure that all the HTML-based templates (including epub)
contain the CSS for columns.
* Columns default to 50% width unless they are given a width
attribute.
Closes#4028.
We now issue `<div class="line-block">` and include a
default definition for `line-block` in the default
templates, instead of hard-coding a `style` on the
div.
Closes#1623.
All templates now include `code{white-space: pre-wrap}`
and CSS for `q` if `--html-q-tags` is used.
Previously some templates had `pre` and others `pre-wrap`;
the `q` styles were only sometimes included.
See #3485.
